Gamma cameras are used in medicinal applications to monitor the progress or distribution of a .gamma.-ray emitting nuclide introduced into a patient. The camera is located adjacent the part or organ of the patient concerned, for instance the brain or liver, and the distribution of the nuclide therein is indicated by the activity at various positions within the organ recorded by the camera.
The gamma camera comprises a .gamma.-ray sensitive crystal which gives a plurality of responses representing particular positions, and related to the position distribution and intensity of the .gamma.-ray emitting nuclide in the patient. A multielement collimator in front of the camera is used to view the patient and direct radiation to corresponding parts of the camera during testing.
In order to render this so called diagnostic scintigraphy accurate, it is essential that the camera or scanner be accurately calibrated, so that the non-uniformity in the spatial response of the camera can be allowed for in drawing conclusions from the results of diagnostic tests. Cameras can vary in sensitivity by as much as .+-.15% over their areas, and need to be calibrated daily.
One way of calibrating gamma cameras is to expose them to a uniform activity in the form known as a flood source. This may conveniently comprise a disc containing a uniformly dispersed .gamma.-emitting nuclide located in particular spaced relation to the camera to provide a uniform field, whereupon camera readings indicate the sensitivity of the various parts of the camera.
One of the most popular nuclides used for diagnostic scintigraphy by introduction into a patient is Technetium-99m which has a .gamma.-ray of 140 KeV, but a half life of only six hours. While it would obviously be ideal to calibrate the gamma cameras using this same nuclide as is used in diagnosis, it will be appreciated that with the very short half life of Technetium-99m this is not practical. It is therefore common practice to use a so-called pseudo-standard, and for this purpose it has been known, in the case of Technetium-99m, to use Cobalt-57 which has half life of 270 days and emits .gamma.-rays at 122 and 136 KeV, and also Gadolinium-153, which has a half life of 242 days and .gamma.-ray lines at 99 and 101 KeV.
These nuclides both suffer from disadvantages when used as calibration nuclides for gamma cameras to be used with Technetium-99m. The Cobalt nuclide presents difficulties because 0.2% of high energy .gamma.-radiation is emitted and because additional .gamma.-ray lines are emitted from the common impurities of Cobalt-56 and Cobalt-58. On the other hand, it will be appreciated that the energies of Gadolinium-153 are rather remote from the 140 KeV .gamma.-ray line of Technetium-99m.
We have now discovered that the nuclide Tellurium-123m is particularly suitable for calibrating gamma cameras which are to be used with Technetium-99m. We have discovered that this Tellurium nuclide can be readily prepared to a satisfactory purity in which it offers the advantages that it has a low level of associated high energy .gamma.-radiation, it has a main .gamma.-ray line at 159 KeV, and it has only this single principle .gamma.-ray line. In addition it has a satisfactory half life of 117 days, and the 159 KeV .gamma.-ray line is satisfactorily close to the .gamma.-ray line of Technetium-99m.